With high integration of an LSI (large scale integrated) circuit, internal interconnection is becoming minute and multi-layered. Along with such a tendency, development of a flattening technique on formation of interconnection, and a processing technique for minute interconnection, and maintenance of reliability become important problems. As one of the solutions to these problems, embedded interconnection technique has been investigated. In particular, a copper embedded interconnection technique aiming at high speed operation and low consumption power is receiving attention.
Formation of a copper film by an electroplating method receives attention as a recent method of copper embedding. In this method, a barrier metal layer is formed in a groove or a connecting hole, and then a copper film is formed by an electroplating method using a copper sulfate solution. In this case, a copper film is often formed by a sputtering method or a CVD (chemical vapor deposition) method on the barrier metal layer and used as a glue layer. The electroplating method realizes embedding in a high aspect structure at room temperature.
However, the conventional technique described above involves the following problems. The process of embedding copper in a groove or a connecting hole by electroplating of copper is described below. As shown in FIG. 1A, a concave part 112 comprising the groove and the connecting hole is formed in an interlayer insulating film 111 by an ordinary RIE (reactive ion etching) method. As shown in FIG. 1B, a barrier metal layer 113 is formed on the inner wall of the concave part 112 and on the interlayer insulating film 111 by forming, for example, a titanium film and a titanium nitride film, as a laminated film, in this order from the lower layer, for example, by a sputtering method, and then a glue layer 114 is further formed thereon. At this time, the barrier metal layer 113 and the glue layer 114 are formed at the opening parts of the concave part 112 in the form of overhang.
As shown in FIG. 1C, because the coverage of the barrier metal layer 113 and the glue layer 114 on the concave part 112 does not become 100%, the resistance of the barrier metal layer 113 and the glue layer 114 is increased at these parts. Under the circumstances, when electroplating is conducted by immersing in a copper electroplating solution 121, current concentration occurs at the opening part (shown by arrows in the figure). The rate of the film formation is thus increased at the part at which current concentration occurs. A bubble 115 is formed inside the concave part 112. As a result, a copper film 116 is formed with a void 115 forming inside the concave part 112, as shown in FIG. 1D. In FIG. 1C, the figure is drawn with the upper surface of the interlayer insulating film 111 being downward on the contrary to the other figures.
FIG. 2 is a schematic cross sectional view showing the voids actually formed on producing a copper film by electroplating. As shown in FIG. 2, it has been found that the copper film 116 is grown in the condition that the voids 115 are formed over the interior to the upper part of the concave parts (grooves) 112 formed in the interlayer insulating film 111.
In the electroplating apparatus 120 for a wafer currently available as shown in FIG. 3, in order to prevent the back surface of the wafer 110 from contacting with a plating solution (containing copper ions) 121, a face-down structure is employed in that the front surface of the wafer 110 faces the plating solution 121. The plating solution is stored in a plating bath 122, and an anode 123 is provided in the plating solution 121.
In the method described above, there is a case where the plating solution 121 cannot be spread into minute parts formed on the surface of the wafer 110 as shown in FIG. 4A. That is, there is a case where a bubble 117 remains inside the concave part (for example, a groove) 112. When electroplating is conducted under such conditions, the copper film 116 is grown in the condition in that the bubble 117 remains and a void 115 is formed inside the concave part 112, as shown in FIG. 4B.
It has been reported by Y. Harada, et al. in Preprints of 58th Shuki Gakujutu Koenkai of the Japan Society of Applied Physics, 3p-E-4, p. 776 (1997) that the void thus formed is avoided by subjecting to a heat treatment at about 400.degree. C. However, a void generated by forming a film by electroplating contains air as different from a void generated by sputtering under high vacuum. Since the air contains about 20% of oxygen, there is a possibility that the surroundings of the void are oxidized, and increase in resistance and deterioration of reliability may occur.